How much does freebore affect pressure in 6xc

[Shawnba67]

I just put together a 6xc, and its throated fairly short.
My length to ogive with Barts 105 Gungnirs is 2.130@ touch.
My Barts measured .666 base to ogive berger 105s were .677 for comparison.

Chamber is .270 brass turned to .2675
Jamming/soft seating bullets im getting an ejector hole mark at 37.5gr of H4350 on brand new Norma brass. No chrono yet was using my new barrel break in rounds to check for pressure to avoid loading a bunch of useless rounds to take to range. ( very glad i did)
ANYWAY 37.5 seems awful low of a MAX charge to me? Does it add up right for anyone else? I read of guys running hair over 40gr!
If it matters trued 700 ptg bolt small pin hole. 30" 8 twist tube

 

[STOMP442]

If it shoots good I would run it through a Chronograph and see what it's running. If you are seeing pressure I am willing to bet you are seeing a proportional velocity. If you are happy with the way it shoots and the velocity you are getting I wouldn't worry too much about freebore.

 

[gambleone]

Did you notice any case head expansion compared to your unfired brass? Seeing that you had ejector swipe, may have had some expansion. Had some norma dasher brass that couldn't go above 31.3 gr of varget without expansion and loose primer pockets, so not sure it was anything to do with your freebore.

 

[Shawnba67]

I did measure for CHE and measured some, but I wax virgin brass with easy extraction.
I fired a round in fireformed Hornady brass and it too had a extractor mark. No smearing just a nice vlean little raised circle. Tommorow I will try to chronograph the 37gr loads that do not leave any ejector marks. That will be relavent info.

 

[JBT47]

I ran 39.6gr H4350 in my XC. Your charge seems very low.

 

[atombomb]

My go to with DTACs is 38.6g of H4350...

 

[tvaught]

I read .270 chamber nk and .2675 loaded nk.

I'm curious what 2 or 3 cases turned to a .265 to .266 loaded nk would do over the chrono and if those still had case head ejector plunger flow.

 

[Shawnba67]

I read .270 chamber nk and .2675 loaded nk.

I'm curious what 2 or 3 cases turned to a .265 to .266 loaded nk would do over the chrono and if those still had case head ejector plunger flow.

They seem to at .266, I had that thought to.

 

[fatelvis]

I ran your cartridge and charge in Quickload, using the Berger 105 Hybrid in place of the Barts bullet (Quickload doesnt seem to list Bart's) and even when it is jammed into the rifling, (10,826 psi start pressure), it's only producing 55097 psi. It shouldn't be leaving ejector marks at that pressure. What is the h2O volume of your particular brass? I can plug that in and hopefully get a better picture of what is going on.

 

[Shawnba67]

I ran your cartridge and charge in Quickload, using the Berger 105 Hybrid in place of the Barts bullet (Quickload doesnt seem to list Bart's) and even when it is jammed into the rifling, (10,826 psi start pressure), it's only producing 55097 psi. It shouldn't be leaving ejector marks at that pressure. What is the h2O volume of your particular brass? I can plug that in and hopefully get a better picture of what is going on.

Thank you very much!
Norma brass weighs 160.6gr
Fired case holds 49.5gr water to top of neck.
Velocity@9ft from muzzle with 37gr h4350
Cci br2 primer 2876fps.ave of 4 shots es was 6fps.
Using a berger 105 in formed hornady brass and 38gr yielded 3009fps slight ejector mark. (Did not check capacity, my 5yr got real interested and his help was, well...helpful! )
Im running about .0005-.001 neck tension and letting the bolt seat the rounds. I plan to try a jump but regardless I should be faster than a 22lr necked to 6mm!
OAL TO ogive with Barts is 2.135 berger is likely 2.145ish.